CHAPTER II. — SPECIAL PHYSIOLOGY OF THE NUTRITIVE FDNCTIONS. 719 



out represent the energy which was used by the plant in con- 

 structing the organic substance of which the wood and the coal 

 consist, and which exists in them as the potential energy of 

 chemical combination. To put the matter more definitely ; if the 

 organic substance which has been formed by a green plant under 

 the influence of light according to the equation 



C02 + HoO = CH20 + 02 

 be burned, the chemical process is precisely reversed, according 

 to the equation 



CH20 + 02 = C02 + H20, 

 and precisely the same amount of energy is evolved in the kinetic 

 state in the second process, as was stored up in the potential state 

 in the first. 



It is, generally speaking, only plants possessing chlorophyll, 

 which can produce organic substance. Inasmuch, therefore, as/ 

 organisms, whether plants or animals, which do not possess chloro- 

 phyll require for their nutrition more or less complex organic 

 substances, they are entirely dependent for their food upon organ- 

 isms which do possess chlorophyll. 



To this general rule exceptions are offered by certain Schizomycetes. Thus ^ 

 some Bacteria {e.g. B. photometricum) contain a purple colouring-matter 

 (bacterio-purpurin), but no chlorophyll: they are, however, capable of forming 

 organic matter with evolution of oxygen when exposed to light ; the bacterio- 

 purpurin appears to perform essentially the same, physical function as 

 chlorophyll, though it does nol^ absorb the same rays of light. Again, other 

 Bacteria, such as the Sulphur-bacteria (Snlphobacteria), the Iron-bacteria {FerrO' 

 bacteria), and the nitrifying Bacteria (Nitrobacteria) produce organic substance, 

 although they possess no chlorophyll, and do so quite independently of light, 

 the necessary energy being obtained in other ways (see p. 731). 



This process is also of great importance in another direction. 

 All living organisms, speaking generally, absorb free oxygen and 

 evolve carbon dioxide in respiration. Those organisms which 

 possess chlorophyll prevent the excessive accumulation of carbon 

 dioxide in the atmosphere, and keep up the supply of free oxygen, 

 in that, under the influence of light, they absorb the former gas 

 from the air, and replace it by an equal volume of the latter. 



The characteristic difference between the a nabo lic capacity of 

 plants which do and of those which do not possess chlorophyll is 

 then this, that the former can produce, from carbon dioxide and 

 water, assimilable or plastic substances containing the elements 

 C, H, and O, whereas the latter cannot produce these, but must 



v. s. B. 3 a 



